DESCRIPTION:(adapted from applicant's abstract) The long term goal of this group is to understand the neural and biomechanical basis for the control of posture and balance. The goal of this application is to clarify the roll of vestibular and neck proprioceptive information and to determine how these two sensory signals are integrated for the control of posture and balance. These experiments will quantify postural control in the absence of either vestibular or neck input. The specific aims are: 1) to determine the contributions of vestibular and neck afferents to postural adjustments that accompany voluntary head movements during stance. Vestibular lesions result in instability and ataxia during head turning. The investigators hypothesize that in the absence of vestibular or neck afferent input subjects fall because they actively destabilize themselves because the nervous system misinterprets the remaining sensory information about position of the trunk in space. 2) To determine the contributions of vestibular and neck afferents to postural orientation during unpredictable disturbances of stance. The investigators hypothesize that without vestibular or neck proprioceptive input subjects will be unable to orient to earth vertical during unpredictable rotations of the support surface and will therefore have difficulty maintaining balance. These studies will provide important new information about the integration of two sensory systems, vestibular and neck proprioception, in the control of balance during stance. Furthermore, these studies will shed new light on the sensorimotor deficits associated with vestibular disorders and with cervical ataxia, and may suggest new approaches to rehabilitation for patients with these disorders.